To kill bacteria, the required UV fluence is about 40 mJ/cm2. A rough estimate shows that 1 W of LED power could disinfect the water flowing out of a fully open faucet. Currently, a single LED has a power of 100 mW, so ten LEDs would be sufficient.
Some companies are already starting to offer such point-of-use disinfection. In the future, these systems could be installed in a rural village to sterilize well water. And assuming that efficiencies continue to improve, the LEDs could get all their energy from a small battery or solar panel.
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Correction (9 September 2021): A previous version of the story incorrectly referred to laser fluence as intensity.
